Washington &
New York –
Phoenix, NASA’s
new m
artian probe, is on schedule
to land on the red planet May 25. But it is the spacecraft’s descent and landing that have scientists biting their nails.


“This is not a trip to grandma’s house,” said Ed Weiler, NASA’s associate administrator for science
. “Putting a spacecraft safely on Mars is hard and risky.”


Phoenix managers refer to the probe’s descent as “
seven minutes of terror” that will define the future of the spacecraft’s $420-million mission. The robotic arm-equipped spacecraft is due to land near the m
artian north pole
to study nearby water-ice and determine if the region was once habitable for primitive life.


“Hopefully the outcome will be different from the Mars Polar Lander outcome,” said Rob Grover, NASA engineer at the Jet Propulsion Laboratory in Pasadena, Calif.


Mars Polar Lander (MPL) entered the martian atmosphere near the planet’s south pole in 1999, but a software glitch caused a premature shutdown of the spacecraft’s engines. It crashed while falling at 80 kilometers per hour, instead of making a soft landing. NASA has worked since then to ensure Phoenix doesn’t suffer the same fate.


No. 1 cause was the faulty indicator on the touchdown sensor,” Grover told Space News
, adding that the sensor falsely told the MPL that it
already had landed.


since have corrected the software issue and made the overall system more robust to avoid future errors.


“We feel like we have adequately tested this vehicle,” Phoenix project manager Barry Goldstein said in a May 13
mission briefing, but added that there is always room for the unexpected. “We fire 26 pyrotechnic events in the last 14 minutes of this vehicle, and every one of those has to go off as planned … We’re very hopeful for success on the 25th.”


Phoenix launched last August and has traveled about
679 million kilometers
to reach the red planet. A planned maneuver to adjust Phoenix’s course was canceled May 10
because the spacecraft is on track for its May 25 landing,
Goldstein said
. At press time,
another maneuver had been scheduled

May 17


NASA also released enhanced images of Phoenix’s landing site, located at 68 degrees north latitude, 233 degrees east longitude in Vastitas Borealis, the northern arctic planes of Mars. This latitude corresponds to northern Canada, just below the Arctic sea, said Phoenix principal investigator Peter Smith of the University of Arizona, Tucson.


Below the surface layer of dust in these plains lies a layer of water-ice mixed with sand and dust. During its three-month mission, Phoenix will use its
robotic arm to dig up samples of this dirty ice and analyze it with onboard science instruments to shed light on the history of water in the m
artian arctic and see if the icy soil could support life.

Risky Landing

Unlike with
MPL, Phoenix project managers will be able to track and communicate with the spacecraft even after it enters the m
artian atmosphere.

A wraparound antenna
on Phoenix’s back
will transmit
an ultra-high frequency signal to Earth via NASA’s Mars Reconnaissance Orbiter (MRO) or Mars Odyssey spacecraft. Europe’s Mars Express orbiter is also on call in case of an emergency, mission managers said.


For Phoenix Project Manager Barry Goldstein, it is the three-second communications gap between Phoenix’s departure from its cruise stage and the first signals to its relay network that gives him the shivers. If Phoenix fails to land successfully, any signals just before landing will prove vital in learning its fate, he said.


Phoenix, like MPL, will
into the m
artian atmosphere at about
21,000 kilometers per hour,
similar to the respective 2004 descents of NASA’s Spirit and Opportunity rovers.
But Phoenix’s arrival would mark the first powered landing on Mars since NASA’s Viking missions of the 1970s. The probe combines new technology with proven methods for landing, including an Apollo-era Earth entry software algorithm to guide the spacecraft’s early descent into the m
artian atmosphere.


A Viking-era parachute is designed to open once Phoenix falls within
12.6 kilometers
above Mars, creating drag to slow the spacecraft as it screams through the atmosphere at supersonic speed. The probe’s landing radar should begin giving altitude and velocity of descent as Phoenix nears the surface, so that the onboard computer can make any necessary landing adjustments.


Two minutes after the parachute deployment, Phoenix will have descended to approximately
1 kilometer
above the surface. The lander should then jettison its backshell and freefall for half a second before lighting up its engines.


Nine of the
12 engines will pulse furiously 10 times per second – an effect Grover likened to “coming down on a jackhammer.” The three non-pulsing engines should fire steadily to help ensure added stability.


“Just before touchdown, we actually pirouette the vehicle …
to maximize solar exposure,” Goldstein said

Navigators at JPL can upload fresh orders to Phoenix’s guidance computer up to three hours before landing, in case course adjustments are required. However, Grover and other NASA engineers will only be able to stand by and trust in their spacecraft technology once the Mars lander begins its descent.


“We’ve done all that’s humanly possible,” Grover said.

Comments: athompson@imaginova.com, jhsu@imaginova.com